hace+period of time+ que

Hi all!
I have learned that hace + period of time + que + present tense tells how long something has been going on, so:
hace dos dias que hago el proyecto significa 'i have been doing the project for two days'
can it also translate as: it has been two days since i did the project? If so, is that considered slang?
Gracias!

The first part of your question was answered by Pliscapoivre. Let me take a stab at the second:

Slang is, according to the dictionary, informal language consisting of words and expressions that are not considered appropriate for formal occasions; often vituperative or vulgar. I don't necessarily consider slang offensive or inapproriate; I think of it as language or expressions that constantly evolve and change, like expressions that teenages use that vary with each generation...for example, "That's 'the bees knees'/'groovy'/'cool'/'hot'/'bad"... each generation created its own way to say that something is great.
Idioms are defined as expressions whose meanings cannot be inferred from the meanings of the words that make it up. In other words, they are expressions for which there are no direct translations. For example, "I lost my shirt" is an idiom that has been around for generations and it is appropriate for use in formal and informal settings, but one cannot translate it directly...it rarely means the person misplaced his shirt...it means he lost everything, right?

So I would consider the expressions of time that you mentioned above idiomatic rather than slang...they are considered common usage, and the meanings do not change over time.